Congestion management in managed packet-switched networks

ABSTRACT

A packet switched network includes a network manager for managing resources on the network and a plurality of switch nodes. A congestion indicator threshold is (Cir) for each resource at each switch node and congestion event indicator is forwarded to the network manager immediately upon the congestion level of a monitored resource exceeding its congestion threshold. A congestion status for the monitored resources of each switch node is maintained by the network manager.

This invention relates to congestion management in managedpacket-switched networks, such as frame relay networks, and encompassesboth the real-time aspects of congestion control and avoidance, andOperations, Administrations and Maintenance procedures to detect theonset of congestion.

Congestion management for frame relay switched networks, which aredesigned to handle bursty traffic, is an important issue due to thepossibility that frame relay resources will become depleted, causingcongestion. In this case congestion management includes real-timemechanisms to prevent or recover from congestion, as well as proceduresto detect the onset of congestion.

Congestion management procedures have several important requirements.The number of frame discards and level of degradation of service to userdevices during congestion must be minimized. They must simple toimplement and place little overhead upon the network, be fair inallocating resources among competing PVCs, limit the spread ofcongestion to other network elements, and optimize the use of networkresources. The configuration complexity should be minimized, yet beflexible enough to allow tuning of the network congestion procedures.Discard frames with the Discard Eligibility bit (DE) set should bediscarded in preference to other frames during a congestion situation.Facilities that notify severely congested circuits should be providedand they should provide sufficient information so that network operatorscan isolate the source of congestion.

Existing practice as implemented, for example, on the Newbridge NetworksCorporation MainStreet product line uses several mechanisms shown inFIG. 1. These include negotiating a minimum bandwidth (CIR) among otherclass of service parameters for each PVC; and monitoring the actualbandwidth utilization at ingress, which involves policing the incominginformation rate and marking traffic which exceeds CIR in support of twolevels of priority and fairness; monitoring the depletion of networkresources; initiating standards-based implicit congestion mechanisms,also referred to as congestion avoidance procedures, during times of lowlevels of congestion; and turning to standards-based explicit congestionmechanisms to efficiently and effectively recover from congestion duringtimes of high levels of congestion.

The above mechanisms generate congestion alarms to reflect the severe,mild and non-congestion state that can exist within a frame, streamand/or the whole card. Alarms are generated and logged to the NewbridgeNetworks Corporation 46020 network management system if the circuit/cardenters the RED congested state. These indicators are debounced by a lowpassing filter to manage the transition between the states as shown inFIG. 2.

Because of the low pass filter implementation, spikes shown in FIG. 2are not accounted for when generating the alarm. This causes loss offrames without an actual indicator of this happening. For a congestionalarm to be raised, the frame stream buffer must be over the SevereCongestion Threshold (SCT) for five continuous seconds. If the bufferutilization dips below SCT the counter is reset to zero regardless ofthe time spent in the SCT range. When in Severe Congestion it isreferred to as being in the RED state. The following statistics are alsosubject to the five second filter: Number of RED alarms, Percentage oftime spent in RED state, Duration of longest RED state.

The RED state is significant because in this state Frames with the DEbit set are discarded. The discarding of flames is not subject to thefive second delay. There are therefore two definitions of the RED state,one for statistics and alarms (five second delay) and one for discards(instantaneous).

For a traffic pattern shown in FIG. 3, the system is in the discardingRED State for 98% of the time (4.9s/5.0s). However the statistical andalarming RED state was never reached. Therefore no alarms are raised andthere is no indication that a RED state has been entered, whilesignificant frames have been discarded without any operator alert.

At first glance, the traffic profile in FIG. 3 may seem very improbable,and unrealistic. In actual practice a similar profile with identicalcharacteristics and the resulting discards and alarms is very common. Itis actually the intentional result of the frame relay congestionavoidance scheme.

The traffic profile in FIG. 4 is a profile that results from theexisting frame relay congestion avoidance scheme. When the switchbuffers reach the SCT level, all the frames with DE set are discarded.This results in the buffer utilization dropping instantly below thethreshold. The buffer then fills up again with the same result. Whileless time is spent in the discarding RED state than in FIG. 3, there arestill a significant number of of frames (all frames with DE set at thetime the threshold is exceeded) discarded without any alarms orstatistics to reflect the reason for their being discarded.

The actual amount of frames discarded will be a function of the numberof frames that have DE set. The more frames with DE set, the morediscards that will occur. This is shown in FIG. 5.

The number of frames with DE set (if all setting is done by NewbridgeNetworks Corporation FREs/FRSs [Frame Relay Engines and Frame RelaySwitches) and not the end devices] will be determined by how “tightly”the network is engineered. In a “loosely” engineered network, where theClass of Service (COS) parameters CIR, Bc and Be are significantlygreater than the actual traffic, there will not be many DE bits setsince CIR and Bc are rarely exceeded. Since relatively few frames arediscarded, the buffer may not fall below the SCT threshold, the fivesecond window will expire, and the alarms will be raised and thestatistics collected.

In a tightly engineered network where the COS parameters accuratelyreflect the traffic profiles, there may be many DE bits set. Since manyframes may be discarded, the buffer may always fall below the SCT levelresetting the five second counter. Therefore the tightly engineerednetwork may discard more frames (more frames with DE set) and never getany alarms.

According to the present invention there is provided a method ofmanaging a packet switched network comprising a plurality of switchnodes and a network manager and wherein packets are discarded in theevent of congestion in the network. A congestion event indicator isnotified to the network manager immediately whenever the congestionlevel of a resource in a switch node in the network exceeds a congestionthreshold, and the network manager maintains a congestion status for theswitch node based on the receipt of said congestion event indicators.

The invention addresses the network requirements with regards to theenhanced frame relay network congestion indicators. These indicators canbe monitored by a network management entity in the generation of troubletickets.

The frame relay engine supports a mechanism for notifying congestion andthe occurrence of frame discards on the switch. When the switch bufferusage or a frame Networks Corporation 46020 will be notified. CPUcongestion will not cause a ‘Congestion Indication’ to be sent to thenetwork manager.

Preferably, a single congestion indicator event will be sent to thenetwork manager when a resource on the switch exceeds a CongestionIndicator Threshold (CIT). The Congestion Indicator Threshold can be setequal to ACT, SCT or MCT for each resource on the switch. Whenever aframe is to be placed in a resource (e.g. transmit queue for a framestream) that has reached or exceeded the Congestion Indicator Thresholdthen the resource is declared as congested. The Congestion Indicatorstatus is updated to indicate that the switch has a congested resource,and an event is sent to the network manager. The status of theCongestion Indicator is changed to ‘No Congestion’ only if the level ofcongestion of all resources on the switch remain below the ‘CongestionIndicator Threshold’ for a period of a user configurable ‘CongestionIndicator Clear Time’.

The invention will now be described in more detail, by way of example,only with reference to the accompanying drawings, in which:

FIG. 1 shows a Newbridge Frame Relay Congestion Management Model;

FIG. 2 shows Congestion Status Thresholds;

FIG. 3 is one example Traffic Profile;

FIG. 4 is another Example Traffic Profile;

FIG. 5 shows the Frame Discard Level Dependancy on # Frames with DE BitSet; and

FIG. 6 is an Example Congestion Indicator Event Notification inaccordance with the invention.

Referring now to the example shown in FIG. 6, the congestion level offrame stream #1 goes above the Congestion Indication Threshold (CIT) onthe switch. A Congestion Indication event is immediately sent to thenetwork manager. Shortly afterwards the congestion level of frame stream#2 goes above the CIT. Both streams 1&2 will be indicated as congestedby the NMTI Node Management Terminal Interface interface.

When the congestion level of all resources on the switch (in our examplestream 1&2) drops below the CIT for a user configurable CongestionIndicator Clear Time, a Clear Congestion Indicator event is sent to thenetwork manager clearing the switch congestion.

An option is available to turn off this congestion indication reportingmechanism. As well, the user is able to clear the historical record ofthe peak level of congestion of the resources.

This feature has no anticipated impact on the traffic flow, when thenetwork includes nodes do not support this feature. The information inthis case could be retrieved by using other tools (stats).

The frame relay network supports a mechanism for notifying the managerof frame discards due to congestion. If congestion event reporting is“on”, this information will be used by the network manager to raisetrouble tickets when frames are discarded due to congestion in thenetwork. An option will be available to turn “off” this congestion eventreporting mechanism, with the default being “off”.

A single congestion indicator event will be sent to the network managerwhen a resource (stream or switch) on the switch exceeds a CongestionIndicator Threshold (CIT). The user can tune the sensitivity andduration of the congestion reporting mechanism by adjusting theCongestion Indicator Threshold and the congestion indicator clearingtime. The user can also adjust the ACT (Absolute Congestion Threshold)excluding switch level, SCT and MCT (mild Congestion Threshold)thresholds for each resource on the card.

The user will be able to set the following options on a per switch levelbasis to tune the reporting mechanism:

Parameter Default Description Congestion Indicator Clear Time 15 1Minute.. 24 Hours Minutes Congestion Indicator Threshold SCT MCT, SCT,ACT Congestion Event Reporting Off On, Off

The network manager can be used to read the status of each resource onthe switch or the record of the peak level of congestion for eachresource since the last reset by the user.

The network manager can also ‘highlight’ congestion status (using amechanism other than trouble ticket) based on a switch congestion level,or actual resource congestion levels.

Currently, this invention can be implemented on any system that has toindicate the status of a resource or device to a central managementstation, particularly, where the resource status changes frequently, andthere is a requirement that the central management station not be overburdened with status change indications from the network.

This invention is directly applicable to LAN or Frame relay switchingequipment that is controlled and monitored from a central networkmanagement station. Resources on the frame relay switch can enter andexit the congested state frequently. This information needs to bevisible from the network station and must represent the peak resourcecongestion levels experienced by the frame relay traffic.

What is claimed is:
 1. A method of managing a packet switched networkcomprising a network manager for managing resources on the network, anda plurality of switch nodes, each said switch node having resources tobe monitored and being in communication with said network manager, andwherein packets passing through said switch nodes are discarded in theevent of congestion, comprising the steps of: setting a congestionindicator threshold (CIT) for each said resource at each switch node,said congestion indicator threshold representing a predetermined levelof congestion; forwarding a congestion event indicator to said networkmanager immediately in response to the congestion level of a monitoredresource exceeding the congestion threshold set therefor; andmaintaining at the network manager a congestion status for the monitoredresources of each switch node in response the receipt at said networkmanager of said congestion event indicators.
 2. A method as claimed inclaim 1, wherein said congestion indicator threshold isuser-configurable.
 3. A method as claimed in claim 2, wherein when thecongestion level of all the resources drops below said congestionthreshold for a predetermined period of time, a clear congestionindicator event is sent to said network manager to clear said congestionstatus.
 4. A method as claimed in claim 3, wherein said predeterminedperiod of time is user-configurable.
 5. A method as claimed in claim 1,wherein the network manager is notified of packet discards due tocongestion.
 6. A method as claimed in claim 5, wherein said networkmanager raises trouble tickets in response to being notified of packetdiscards.
 7. A method as claimed in any one of claims 1 to 6, whereinsaid packet switched network is a frame relay network.